GLOBAL-LOCAL ITERATIVE METHODS FOR EQUIVALENT DIFFUSION-THEORY PARAMETERS IN NODAL CALCULATION

Abstract

To get the equivalent assemblywise flux weighted constants (FWCs) for use in the coarse-mesh nodal analysis, the global/local iterative homogenization procedures, including the procedures based on variational principles, are developed in this study. The nodal expansion method is employed in the global core analysis and the fine-mesh finite difference method in the local assembly calculation. To achieve fast and stable convergence, the mixed boundary condition is imposed on the assembly surface in local assembly calculation, where we modulate the surface flux using the assemblywise fundamental mode eigenfunction. The results of numerical tests using the assemblywise FWCs are compared with the reference solutions for the PWR and BWR problems. In the PWR problems, there is no strong need for the global/local iterative homogenization. However, in the BWR problems, the iterative procedures we developed remarkably improve the accuracy of the assembly power distribution. Especially the surface flux should be modulated in the local calculation, when there exists high heterogeneity at the node surface as in BWR. The procedures based on the variational principles provide similar trend in accuracy to the direct iterative procedures.